The orthogonal injection of jets into microchannels with laminar crossflow is a common microfluidic operation. It is used for mixing in analytical micro systems (μTAS), chemical micro reactors or emulsification. If parallelized on a large scale, it has potential for fuel premixing in turbo machinery. Furthermore, it can be used as a transistor-like element for flow control or even as a sensor for flow properties. The above mentioned applications rely on the unique flow field induced by jets in confined crossflow: the momentum injected perpendicularly to the crossflow direction is transferred into a counter rotating vortex pair. The jet trajectory is mainly determined by the ratio of momenta and viscosity, resulting in three basic flow modes. With low ratios, the jet is too slow and cannot lift off from the channel bottom (creep flow), while the jet impinges almost undeflected onto the opposite channel wall at large ratios (impingement flow). The third type, core flow, occurs for intermediate ratios, and is characterized by well-defined jet boundaries and a distinctive vortex pair occupying the core domain of the channel. This paper investigates the basic theory of laminar liquid/liquid jets in rectangular microchannels with a characteristic diameter of some hundred microns. The nozzles are of a circular shape with diameters between 50 μm and 150 μm. Two simple models relating geometry and flow properties with the jet trajectory are derived, with particular consideration for the dimensionless characteristics. These analytic predictions are compared to observations for liquid flow. CFD simulations are used to study the impact of the flow mode on heat transfer, as well as the role of the counter rotating vortex pair. Finally, confocal laser microscopy is used for an experimental visualization of the flow field.

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